(57) Process for the preparation of a compound of formula or a salt thereof
 wherein R, represents a hydrogen atom or a group of formula R2SO2 or phenyl-CH(R3)-wherein R2 represents a phenyl, tolyl or C1-4 alkyl group and R3 represents a hydrogen atom or a phenyl or C 1.4 alkyl group, which process comprises contacting nickel in an oxidation state of at
least 3 with a 3-hydroxymethyl azetidine derivative of formula
 wherein R4 represents a group of formula R2SO2 or phenyl-CH(R3)-and R5 represents a hydrogen atom or a hydroxymethyl group or a group of formula COOH or
a salt thereof, followed, where R5 is not a hydrogen atom, by the decarboxylation of the compound of the 3,3-dicarboxylic
intermediate product or a salt thereof and. if desired to produce a compound in which
R. represents a hydrogen atom, by deprotection of the N-atom. Also. novel N-substituted
azetidine carboxylate derivatives of the formula
 wherein X represents a group CH2OH or COOY where Y represents a hydrogen or an alkali or alkaline earth metal atom.
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